1. Field of the Invention
The present invention relates to a DC-DC converter that is highly efficient, small, and inexpensive.
2. Description of the Related Art
FIG. 1 shows a DC-DC converter according to a related art. In FIG. 1, a DC power source E has two ends connected to a series circuit consisting of switch elements Q1 and Q2. The switch elements Q1 and Q2 are, for example, MOSFETs. Between drain and source of the switch element Q1, a series circuit consisting of a primary winding P1 of a transformer T1 and a current resonant capacitor C2 is connected in parallel with the switch element Q1. The primary winding P1 involves an exciting inductance Lp and a leakage inductance Lr. In parallel with the switch element Q1, there is connected a voltage resonant capacitor C1.
Secondary windings S1 and S2 of the transformer T1 are connected in series with each other. The secondary windings S1 and S2 are connected to anodes of diodes D51 and D52, respectively, in opposite polarities. Cathodes of the diodes D51 and D52 are connected to each other and through a positive pole of a smoothing capacitor C51 to an output terminal +OUT. A connection point of the secondary windings S1 and S2 is connected through a negative pole of the smoothing capacitor C51 to an output terminal −OUT.
Between the output terminals +OUT and −OUT, there is connected a voltage detector 51. An output terminal of the voltage detector 51 is connected through a light emitting diode PC-a of a photocoupler to the output terminal +OUT. A phototransistor PC-b of the photocoupler is connected to an input terminal of a variable-frequency oscillator 41 as a voltage controlled oscillator that changes an oscillation frequency according to a voltage detected by the voltage detector 51.
An output terminal of the variable-frequency oscillator 41 is connected to a clock terminal of a flip-flop 43. The flip-flop 43 has exclusive two outputs that are connected to dead time circuits 45a and 45b, respectively.
An output of the dead time circuit 45a is connected through a driver 49a or a buffer to a gate terminal of the switch element Q1. An output of the dead time circuit 45b is connected to a level shifter 47. The level shifter 47 is connected through a driver 49b or a buffer to a gate terminal of the switch element Q2.
Operation of the DC-DC converter according to the related art having the above-mentioned configuration will be explained with reference to FIGS. 1 and 2.
The variable-frequency oscillator 41 sends a clock signal to the flip-flop 43, which outputs exclusive two signals each having a duty of 50%. One of the output signals is supplied to the dead time circuit 45a, which adds a dead time to the signal. The dead time added signal is passed through the driver 49a to become a drive signal for the switch element Q1.
The other output signal from the flip-flop 43 is supplied to the dead time circuit 45b, which adds a dead time to the signal. The dead time added signal is sent to the level shifter 47 which changes the voltage of the signal to a higher level. The high-voltage signal is passed through the driver 49b to become a drive signal for the switch element Q2. In response to the drive signals, the switch elements Q1 and Q2 alternately turns on/off interrupted by each dead time.
When the switch element Q2 turns on, a current passes through a path extending along E, Q2, P1, C2, and E. At this time, a voltage applied to the primary winding P1 of the transformer T1 causes the secondary winding S1 to generate a voltage in proportion to a turn ratio. This results in passing a current through a path extending along S1, D51, C51, and S1. At the same time, a load current passes through the smoothing capacitor C51 and output terminals +OUT and −OUT to a load (not illustrated). The load current equivalently passes through a series resonant circuit of the leakage inductance Lr, smoothing capacitor C51, and current resonant capacitor C2, to form a resonant current. A capacitance relationship of the capacitors C51 and C2 can be described such as C51>>C2, and therefore, the resonant current is substantially determined by the leakage inductance Lr and current resonant capacitor C2.
The exciting inductance Lp of the primary winding P1 passes a triangular exciting current to accumulate energy in the transformer T1. As a result, the primary winding P1 passes a current that is a superimposition of the resonant current and exciting current.
When the switch element Q2 turns off, the energy in the transformer T1 accumulated by the exciting inductance Lp is released through a path extending along Lp, C2, C1, Lr, and Lp. Assuming relationships of Lp>>Lr and C2>>C1, and therefore, a resonant current at this time is substantially determined by the exciting inductance Lp and voltage resonant capacitor C1 and a voltage quasi-resonant waveform appears at the voltage resonant capacitor C1 accordingly. Based on the quasi-resonant frequency, the dead time circuit 45a sets a dead time. The voltage of the voltage resonant capacitor C1 serves as a switching voltage of the switch elements Q1 and Q2, so that the switch element Q1 realizes a zero-volt switching operation. Once the set dead time elapses, the switch element Q1 is turned on.
When the switch element Q1 turns on, the electromotive force of the current resonant capacitor C2 passes a current IQ1 through a path extending along C2, P1, Q1, and C2. At this time, a voltage applied to the primary winding P1 causes the secondary winding S2 to generate a voltage in proportion to a turn ratio. The electromotive force of the secondary winding S2 passes a current IS2 through a path extending along S2, D52, C51, and S2. At the same time, a load current passes through the smoothing capacitor C51 and output terminals +OUT and −OUT to the load (not illustrated). The load current equivalently passes through the series resonant circuit of the leakage inductance Lr, smoothing capacitor C51, and current resonant capacitor C2, to form a resonant current. The capacitors C51 and C2 have a capacitance relationship of C51>>C2, and therefore, the resonant current is substantially determined by the leakage inductance Lr and current resonant capacitor C2.
The exciting inductance Lp of the primary winding P1 passes a triangular exciting current to accumulate energy in the transformer T1. As a result, the primary winding P1 passes a current that is a superimposition of the resonant current and exciting current.
When the switch element Q1 turns off, the energy in the transformer T1 accumulated by the exciting inductance Lp is released through a path extending along Lp, Lr, C1, C2, and Lp. There are relationships of Lp>>Lr and C2>>C1, and therefore, a resonant current at this time is substantially determined by the exciting inductance Lp and voltage resonant capacitor C1 and a voltage quasi-resonant waveform appears at the voltage resonant capacitor C1 accordingly. Based on the quasi-resonant frequency, the dead time circuit 45b sets a dead time. The voltage of the voltage resonant capacitor C1 serves as a switching voltage of the switch elements Q1 and Q2, so that the switch element Q2 realizes a zero-volt switching operation. Once the set dead time elapses, the switch element Q2 is again turned on. Thereafter, the above-mentioned actions are repeated.
The voltage detector 51 detects an output voltage appearing at the output terminals +OUT and −OUT, generates an error voltage signal according to a difference between the output voltage and a reference voltage, and outputs the error voltage signal from the diode PC-a of the photocoupler. The diode PC-a of the photocoupler transmits in an insulated state the error voltage signal to the primary side. Receiving the error voltage signal, the transistor PC-b of the photocoupler variably controls the oscillation frequency of the variable frequency oscillator 41. Variably controlling the oscillation frequency of the oscillator 41 results in controlling the output voltage. Namely, increasing the oscillation frequency results in decreasing the output voltage and decreasing the oscillation frequency results in increasing the output voltage.